1. Field of the Invention
The present invention relates to a method of producing thin-film single-crystal devices, solar cell modules, and a method of producing the same. The thin-film single-crystal devices include, for example, photoelectric converters such as solar cells and devices for a circuit for driving image display elements such as liquid crystal display elements.
2. Related Background Art
A solar cell is becoming in common use as an independent source for driving various types of electrical machinery and apparatus or as a source for system interconnection with commercial electric power. As semiconductors constituting a solar cell, silicon and gallium arsenide are generally used. In order to obtain a high photoelectric conversion efficiency (efficiency to convert optical energy into electric power), these semiconductor single-crystals are preferably used.
In large-area image display elements such as liquid crystal display elements, with an increasing demand for a more high-fine and high-speed image display in recent years, a drive circuit formed within an element is required to have much higher capability. In order to meet this demand, the drive circuit should be formed on single-crystal silicon rather than amorphous or polycrystal silicon.
When using single-crystal semiconductors for the above purpose, there arise several problems. For example, in cases where silicon is used in a solar cell, the thickness of the single-crystal wafers commonly used is as thick as about 300 to 600 xcexcm, while the film thickness required for the absorption of the incident sunlight is about 30 to 50 xcexcm. Under recent circumstances where the single-crystal silicon used in solar cells accounts for ten percent of the total production, its consumption should be reduced. In image display devices, because of the form in which they are used, the light must be transmitted through the areas among the elements in a drive circuit. However, the single-crystal wafers in a common use are difficult to have such a structure formed thereon. In addition, the thickness of a single-crystal layer required for the drive element itself is only 1 xcexcm or less, the rest portion merely serves as a supporting substrate.
In order to solve this problem, thin-film single-crystals having a suitable thickness should be selected depending on the purpose for which it is used; however, as long as the prior arts are employed, a single-crystal layer having a thickness of 300 xcexcm or less is difficult to produce. Specifically, in some methods of prior art, since single-crystal substrates are produced in such a manner as to slice and polish an ingot single-crystal obtained by subjecting a melt of crystal material to crystal growth, single-crystal of 300 xcexcm or less in thickness are difficult to obtain. In some other methods, in order to obtain a high-quality thin-film single-crystal for special purposes, etching is conducted on the back side of a single-crystal substrate having a thickness of several hundreds xcexcm; however, a high-quality thin-film single-crystal is considerably difficult to produce by these methods. Recently, however, the method disclosed in Japanese Patent Application Laid-Open No. 7-302829 enables the peeling of a thin-film single-crystal from a substrate on which the thin-film single-crystal is epitaxially grown, and a technique disclosed in Japanese Patent Application Laid-Open No. 9-331077 enables the peeling of a portion ranging from the surface of a single-crystal substrate to a certain depth, as a thin-film, from the substrate. These methods, however, also have a problem that lattice defects may appear in a thin-film single-crystal during the peeling operation, leading to a reduction in quality of the thin-film single-crystal, and in an extreme case, cracks appear in the thin-film single-crystal, leading to a remarkable reduction in production yield. Thus, effective solutions of the above problems have been desired.
Roughly speaking, there are two common types of solar cells at present: solar cells using amorphous silicon and solar cells using crystalline silicon. And these solar cells are devised in various ways depending on their applications so as to make full use of their respective characteristics.
For example, amorphous silicon solar cells, which are formed by depositing an amorphous silicon film on a conductive substrate by the plasma CVD method and forming a transparent conductive layer on the amorphous silicon film, are inexpensive, lightweight, and excellent in impact resistance and flexibility compared with solar cells using crystalline silicon. Making good use of these characteristics, attempts have been made to use an amorphous silicon solar cell as a solar cell incorporated with building materials, that is, as an amorphous silicon solar cell incorporated with roof, wall, etc. of building.
In this case, a solar cell is used as a building material by bonding a reinforcing material to its non-light-receiving side via a bonding agent. Bonding a reinforcing material enhances mechanical strength of a solar cell module and prevents warps and strain, due to changes in temperature. This type of solar cell is often installed on a roof because more sunlight can be collected there. In its use as a solar cell incorporated with roofing, conventionally the installation has been performed as follows: fitting a frame to the solar cell, installing a stand on a roof, and installing the solar cell on the stand. On the other hand, the solar cell with a reinforcing material bonded thereto can be directly installed on a roof as a roofing material by bending the reinforcing material. This allows to materially reduce the raw material cost as well as the number of operational steps, and hence to provide a roof with solar cells at a low price.
In addition, the solar cell can be made lightweight since it requires neither frame nor stand. Thus, the solar cell can be treated as a metal roofing, which has lately attracted considerable attention, due to its excellent workability, lightweight and superior earthquake resistance.
The solar cell module incorporated with roofing, for example, disclosed in Japanese Patent Application Laid-Open No. 7-302924 is excellent in workability since the portions where roof materials engage with each other (the region where photovoltaic elements are not arranged) have been subjected to bending just like ordinary roofing. It is also easy to handle in terms of machining since the current molding machine used for ordinary roofing is applicable as it is. It enables the installation of a roof with solar cells at low costs.
As described above, since it is preferable that the solar cell module incorporated with roofing is constructed in such a manner as to be lightweight and machinable like ordinary roofing, the most common type of solar cell module incorporated with roofing has a construction in which a photovoltaic element is bonded to or installed on a steel plate (roofing) and subjected to insulation sealing with resin, as shown in FIGS. 10A and 10B.
FIGS. 10A and 10B are a schematic perspective view of a plate-type solar cell module incorporated with roofing and a cross-sectional view taken along the line 10Bxe2x80x9410B of FIG. 10A, respectively. In FIGS. 10A and 10B, reference numeral 1001 denotes a surface protective material, numeral 1002 a filler material, numeral 1003 a photovoltaic element and numeral 1004 a reinforcing plate.
An amorphous silicon solar cell module, when used as a solar cell module incorporated with roofing described above, has preferable and excellent characteristics, but has problems that its photoelectric conversion efficiency (efficiency to convert optical energy into electric power, hereinafter sometimes refereed to as xe2x80x9cconversion efficiencyxe2x80x9d) is generally low compared with that of a crystalline silicon solar cell and its properties may deteriorate due to light (optical deterioration) to some extent when it is used for a long period of time.
On the other hand, for a crystalline silicon solar cell, its photoelectric conversion efficiency is generally high compared with that of an amorphous silicon solar cell and it is hard to subject to optical deterioration. Making use of these characteristics, there has been developed a crystalline silicon solar panel which enables space saving.
There remain, however, several problems to be solved when using single-crystal silicon in a solar cell module (particularly in a solar cell module incorporated with roofing).
In cases where silicon is used in a solar cell, the thickness of the single-crystal wafers commonly used is as thick as about 300 to 600 xcexcm while the film thickness required for the absorption of incident sunlight is about 30 to 50 xcexcm. Accordingly, if a single-crystal silicon wafer is used as a photoelectric converting layer as it is, it means single-crystal silicon is needlessly consumed.
In the present situation, where the amount of the silicon crystal used in solar cells accounts for 10% or more of its total production, there is an increasing demand for reducing its consumption. Further, in cases where a silicon wafer with thickness described above is used as a photoelectric converting layer of a solar cell as it is, since the solar cell hardly has flexibility which an amorphous silicon solar cell with an thin-film amorphous silicon layer has, it is very hard to fix the silicon wafer as it is, for example, to a curved surface. Thus the shape and installation site of the solar panel are limited, and when intending to use it in a module incorporated with building materials, it will be under many restrictions.
On the other hand, lately there have been demands for building materials and solar cells to have a wide variety of shapes, in terms of their functions and designs. In order to provide building materials and solar cells having a wide variety of shapes under such circumstances, it is hard to remain the shape of a photovoltaic element planar. And machinability should be secured for the whole region of the solar cell, including a photovoltaic element.
As an example of measures to keep up with the trend toward diversification as described above, there is disclosed a corrugated solar cell module, for example, in Japanese Patent Application Laid-Open No. 8-222752, Japanese Patent Application Laid-Open No. 8-222753 and Japanese Patent Publication No. 6-5769. In each case, in order to increase light-using efficiency, a photovoltaic element is arranged in a corrugated form, and the corrugated solar cell module is produced by following the procedure of bonding a photovoltaic element, with an adhesive agent, to a steel plate previously machined in a corrugated form.
It is, however, in amorphous silicon solar cells excellent in flexibility that the application of these techniques are feasible, and the application to crystalline silicon solar cells employing silicon wafers, which are poor in flexibility, is difficult.
One possible solution to these problems, that is, problems an amorphous silicon solar cell and a crystalline solar cell respectively have is to select thin-film single-crystal having a suitable thickness depending on the purpose for which it is used; however, with the prior arts single-crystal having a thickness of 300 xcexcm or less is hard to produce. Specifically, in some prior art methods, since single-crystal substrates are produced in such a manner as to slice and polish an ingot single-crystal obtained by subjecting a melt of crystal material to crystal growth, single-crystal of 300 xcexcm or less thickness are difficult to obtain. In some other methods, in order to obtain high-quality thin-film single-crystal used for special purposes, etching is conducted on the back side of a single-crystal substrate of several hundreds xcexcm thickness; however, its production process is complicated, and hence under many restrictions. Recently, however, there have been proposed methods which make it possible to peel a thin-film single-crystal from a single-crystal substrate; for example, a method disclosed in Japanese Patent Application Laid-Open No. 7-302889 enables the peeling of thin-film single-crystal, which is formed on a single-crystal substrate by the epitaxial growth, from the substrate, and a technique disclosed in Japanese Patent Application Laid-Open No. 9-331077 enables the peeling of a portion ranging from the surface of a single-crystal substrate to a certain depth, as a thin film, from the substrate.
These types of thin-film single-crystal can be molded into a curved surface form to some extent, since it is excellent in flexibility compared with the currently used silicon wafers as described above, although it is inferior to amorphous silicon thin films. However, even in the thin-film single-crystal, when it is bent carelessly during a peeling step and during the module production process involving bending, lattice defects may appear in it, leading to a reduction in its quality, in an extreme case, cracks may appear in the same, leading to a remarkable reduction in its production yield. Furthermore, even when thin-film single-crystal is arranged in the region not subjected to bending, it can be momentarily stressed by wind pressure or vibration depending on the circumstances in which it is used, in addition, it can be subjected to bending due to its deformation with lapse of time, and hence to stress. Thus lattice defects may appear in the thin-film single-crystal, leading to a reduction in its quality, in an extreme case, cracks may appear in the same.
Accordingly, an object of the present invention is to provide, in a case of producing a thin-film single-crystal device, a method of peeling a thin-film single-crystal from a substrate without causing defects and cracks which makes possible to produce a high-quality thin-film single-crystal device in a good yield.
Another object of the present invention is to provide a high-quality thin-film single-crystal solar cell module which solves the aforementioned problems which arise during the use and production of a solar cell module having a thin-film single-crystal as at least one portion thereof and which is excellent in durability and reliability without defect and crack, and it is to provide a method of producing the module.
Specifically, the present invention provides a method of producing a thin-film single-crystal device by utilizing a thin-film single-crystal which is obtained by forming a peeling layer and a thin-film single-crystal on the surface of a substrate in this order, bonding a flexible sheet member to the surface of the above thin-film single-crystal or to the surface of a layer additionally formed on the surface of the above thin-film single-crystal, and peeling the above thin-film single-crystal together with the above sheet member from the substrate by applying force to the above sheet member in such a manner as to curve the sheet member, wherein, the peeling of the thin-film single-crystal from the substrate is carried out in such a manner that the directions of all the straight lines made on the surface of the thin film by the appearance of planes in which the thin-film single-crystal is most apt to cleave are different from the front line of the peeled portion, so as to prevent the generation of defects and cracks.
Further, the present invention provides a solar cell module with flexibility comprising a photovoltaic element having a thin-film single-crystal as at least one portion thereof, wherein the direction in which the above module is inherently apt to flex is different from the direction in which the above thin-film single-crystal is most apt to cleave.
Still further, the present invention provides a solar cell module with flexibility comprising a photovoltaic element having a thin-film single-crystal as at least one portion thereof, wherein at least one portion of a region having the thin-film single-crystal of the above module is subjected to plastic deformation, and wherein the direction of the plastic deformation is different from to the direction in which the above thin-film single-crystal is most apt to cleave.
Further, the present invention provides a method of producing a solar cell module with flexibility comprising a photovoltaic element having a thin-film single-crystal as at least one portion thereof, wherein the above thin-film single-crystal is arranged in such a manner that the direction in which the above module is inherently apt to flex is different from the direction in which the above thin-film single-crystal is most apt to cleave.
Still further, the present invention provides a method of producing a solar cell module with flexibility comprising a photovoltaic element having a thin-film single-crystal as at least one portion thereof, which comprises a step of subjecting at least one portion of a region having the thin-film single-crystal of the above module to plastic deformation, wherein the plastic deformation is carried out in such a manner that the direction of the plastic deformation is different from the direction in which the above thin-film single-crystal is most apt to cleave.
In the present invention, suitably the thin-film single-crystal has the diamond- or zinc blende-type structure.
In the present invention, preferably the thin-film single-crystal is produced by forming a peeling layer and a thin-film single-crystal layer on a substrate in this order, bonding a plate-shaped flexible member to the surface of the above thin-film single-crystal layer or to the surface of a layer additionally formed on the above thin-film single-crystal layer, and peeling the above plate-shaped member from the substrate by applying force to the plate-shaped member in such a manner as to curve the plate-shaped member. The other suitable methods of producing a thin-film single-crystal according to the present invention include, for example, a method including a polishing step and a method including an etching step.
In the present invention, preferably the angle between the direction in which thin-film single-crystal is most apt to cleave and any one of or any two of or all of the front line of a peeled portion, the direction in which the module is inherently apt to flex deflect and the direction of plastic deformation is 5 degrees or larger, more preferably 10 degrees or larger.